Polypropylene is a versatile and widely used thermoplastic. Many of its uses are in applications that take advantage of its excellent physical properties in molded shapes. However, while polypropylene has outstanding stiffness, it fails in a brittle mode on impact, especially at subambient temperatures. Because many applications that could take advantage of the excellent stiffness also require a tougher material, one that will fail in either brittle or brittle ductile mode, softer, more amorphous polymers have traditionally been added to polypropylene to improve polypropylene impact resistance and tensile toughness. Applications such as this include interior trim parts of vehicles, such as trucks and passenger automobiles.
To mitigate the brittle nature of polypropylene, as discussed above, softer, more amorphous polymers are often added to polypropylene, these materials have traditionally included EPR and EPDM. These ethylene .alpha.-olefin co or terpolymers are most often ethylene propylene rubbers or ethylene propylene diene monomer rubber (EP or EPDM respectively). These materials are generally characterized by a heat of melting generally below 30 J/g, although ethylene .alpha.-olefin copolymers including higher .alpha.-olefins such as butene, hexene, octene and the like have been also been used, these materials have low crystallinity, and high extractability, and are generally in the density range 0.86 to 0.900 g/cm3 while conventional EPR or EPDM materials have densities in the range generally at or below 0.86 g/cm3.
More recently, other soft materials have been added to impact modify polypropylene, such as ethylene butene copolymers. In the publication Impact Modification of Polypropylene with Exact.TM. Plastomers, Society of Plastics Engineers, 1994, the author describes advantageous use of such ethylene butene polymers, with an upper density limit of 0.910 g/cm3. Of note in the data presented in this publication is that stiffness or modulus decreases fairly rapidly with increasing plastomer content. Fabricators of automotive parts often desire to have a stiff part, as well as one that is tough, as exemplified by tensile toughness. Soft or elastomeric or amorphous polymers inevitably tend to soften, or make less rigid, the polypropylene polymers that they make tougher. These softer polymers are often characterized by being totally or substantially extractable or soluble in specific solvents. An additional drawback of isotactic polypropylene/ethylene propylene polymers (iPP/EP) or iPP/EPDM blends is cost. EP or EPDM costs considerably more than polypropylene, raising the cost of the blend.
Therefore, there is an unfilled commercial need for a blend constituent with polypropylene that will maintain or improve tensile toughness especially at low temperature or even enhance the low temperature tensile toughness, while minimizing or eliminating the negative effect on stiffness, as measured by modulus.